1. Field of the Invention
The present invention relates to a vibration wave driving apparatus, and more particularly, to a configuration of a vibration element used in a bar-shaped vibration wave driving apparatus.
2. Related Background Art
A bar-shaped vibration wave driving apparatus includes, as a basic structure, a vibration element composed of elastic members made of metal or the like and a piezoelectric element as an electro-mechanical energy conversion element. The bar-shaped vibration wave driving apparatus generates a driving vibration, such as a travelling wave or the like, through application of an alternating voltage (an alternating signal), with different phases, to the piezoelectric element.
A contact member is brought into pressure contact with a frictional surface of the elastic member through a pressurizing means and the contact member is frictionally driven by the driving vibration generated in the frictional surface of the elastic member to allow the vibration element and the contact member to be moved relative to each other.
There is a vibration wave motor, as an example of such a vibration wave driving apparatus, in which a vibration element is used as a stator, and the contact member is a rotor.
Examples of the vibration element of such a vibration wave motor include those with a configuration in which a ring-shaped piezoelectric element plate is attached to one surface of a ring- or disc-shaped elastic member and those of a type in which the rotation of the rotor is taken out through an output shaft or of a type in which rotation of the rotor is taken out directly.
Such a vibration wave motor has been applied to products to be used for driving a camera lens and the like. There are annular type and bar-shaped type vibration wave motors.
FIG. 10A is a structural view of a vibration element of a bar-shaped vibration wave motor used for driving a camera lens. FIG. 10B shows a vibration mode (with the z-axis assigned to the axial direction and the r-axis assigned to the radial direction) in an axis part of the bar-shaped vibration element.
Reference numeral 101 indicates a first elastic member; 102, a second elastic member; and 103, a piezoelectric element. Reference numeral 106 denotes a shaft member passing through the first elastic member 101, the piezoelectric element 103, and the second elastic member 102. One end of the shaft member 106, located on the side of a rotor 110, is fixed to a fitting member 109 to be attached to a product, and the latter end is fixed to a nut 115. A threaded portion is formed in the other end of the shaft member 106. With the nut 115 tightened, the first elastic member 101, the piezoelectric element 103, and the second elastic member 102 disposed between a flange portion provided for the shaft member 106 and the nut 115 are sandwiched and fixed therebetween. Reference numeral 110 indicates the rotor, as described above, and reference numeral 107 denotes a friction member fixed to the first elastic element 101 to be in contact with the rotor.
When a driving signal is applied to the piezoelectric element 103, a bending vibration (e.g., FIG. 10B illustrates a primary bending vibration) is excited in the bar-shaped vibration element, whereby the bar-shaped vibration element makes a swing movement substantially about the z-axis. In this manner, the friction member 107 may be caused to make a circular motion around the z-axis.
It seems that the vibration element of such a bar-shaped vibration wave driving apparatus has been reduced in size in its radial direction, but there is still room for reduction in size in its thrust direction, i.e., in the length of its axis.
However, when the vibration element is simply shortened, there arise problems in that the resonance frequency increases and the vibration displacement is reduced, which causes a deterioration in the efficiency of friction drive, an increase in price of a driving circuit element due to the high frequency, and/or an increase in loss inside the element. Further, when the vibration element is simply made thinner to lower the resonance frequency, the diameters of a piezoelectric element and a frictional surface are also reduced, thus decreasing a generating force of the piezoelectric element and the friction torque. Therefore, it is conceivable that the output of the motor is made small.
As a technique for solving the above problems and shortening the axial length of a bar-shaped vibration wave driving apparatus, FIG. 11 illustrates one disclosed in Japanese Patent Application Laid-open No. 2001-145376.
A vibration apparatus in such document is identical to a conventional product in that a piezoelectric element 203 is sandwiched and fixed between a first elastic member 201 and a second elastic member 202. However, it is different from the conventional product in that the first elastic member 201 with a frictional surface is divided into an inner diameter portion and an outer diameter portion that are connected to each other through a thin connection part 210.
According to this construction, even if the axial length of the bar-shaped elastic member is shortened, a low resonance frequency can be obtained since the first vibration element has a sufficiently high mass.
According to this technique, however, when the connection part 210 is made thin to allow the resonance frequency to be lowered, its rigidity is deteriorated, and displacement generated in the piezoelectric element is absorbed by a soft spring of the connection part 210. Consequently, it is difficult to transmit the driving force to a rotor efficiently. Thus, it seems that there is still room for further improvement.
One aspect of this invention is to provide a vibration wave driving apparatus including an electro-mechanical energy conversion element that is sandwiched and fixed between elastic members, wherein a third elastic member is provided between the electro-mechanical energy conversion element and one of the elastic members. The third elastic member has a diameter larger than that of the electro-mechanical energy conversion element. When a driving vibration is applied to the electro-mechanical energy conversion element, a vibration element excites a bending vibration and this bending vibration allows an out-of-plane bending vibration to be excited in the third elastic member. Since a rotor is brought into contact with the third elastic member sandwiched between the elastic member and the electro-mechanical energy conversion element, the size of the vibration wave driving apparatus can be reduced. In addition, since a travelling wave produced by the bending vibrations of the vibration element and a travelling wave produced by the out-of-plane bending vibrations of the third elastic member are generated at the frictional surface of the vibration element, the output of the vibration wave driving apparatus can be improved.